Formally, given an input image \(I\)  of size \(w \times h\) , let us denote by \(B_k = (x_1, y_1, x_2, y_2)\)  the bottom-left and top-right corners of the \(k\) -th bounding box produced by the student network. Typically, this is achieved by regressing the offset of an anchor box or object proposal. We then make use of a Spatial Transformer [1]} to extract the image region corresponding to \(B_k\) . Specifically, we compute the transformer matrix
\(A_k = \begin{bmatrix}(x_2-x_1)/w & 0 & -1 + (x_1+x_2)/w\\0 & (y_2-y_1)/h & -1 + (y_1+y_2)/h\end{bmatrix}\;,\) 
